Feat: Numbas updated with polymorphic association

[satikaj]

Description

Latest commit introduces polymorphic association to task comments. Currently, task comments can belong to either Scorm test attempts or Overseer assessments.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Unit tests passed. Comments display as usual in the frontend.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if appropriate
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have created or extended unit tests to address my new additions
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[macite]

Looking super close - the Task Comment table is huge, so I think we can optimise the update... see the comment. @satikaj do you have time to make these changes?